LTC1707 LINER [Linear Technology], LTC1707 Datasheet - Page 12

• Manufacturer Part Number: LTC1707
• Description: High Efficiency Monolithic Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1707
Design Example
As a design example, assume the LTC1707 is used in a
single lithium-ion battery-powered cellular phone applica-
tion. The V
down to about 2.85V. The load current requirement is a
maximum of 0.3A but most of the time it will be in standby
mode, requiring only 2mA. Efficiency at both low and high
load currents is important. Output voltage is 2.5V. With
this information we can calculate L using equation (1),
Substituting V
f = 350kHz in equation (3) gives:
12
L
L
††
* SUMIDA CD54-220
†
0.1 F
AVX TPSC107M006R0150
AVX TPSC226M016R0375
f
350
C
47pF
C
SS
ITH
1
IN
kHz
I
L
will be operating from a maximum of 4.2V
2 5
OUT
V
.
OUT
V
120
1
2
3
4
= 2.5V, V
U
I
RUN/SS
V
GND
TH
FB
mA
1
LTC1707
U
V
SYNC/MODE
Figure 8. Single Lithium-Ion to 2.5V/0.3A Regulator from Design Example
V
OUT
1
IN
IN
= 4.2V, I
2 5
4 2
V
SW
REF
V
.
.
IN
V
V
W
8
7
6
5
22 H*
24 1
L
80.6k
169k
= 120mA and
1%
1%
R1
.
R2
U
H
+
C
100 F
6.3V
OUT
(3)
†
V
2.5V
0.3A
OUT
1707 F08a
+
A 22 H inductor works well for this application. For best
efficiency choose a 1A inductor with less than 0.25
series resistance.
C
temperature and C
0.25 . In most applications, the requirements for these
capacitors are fairly similar.
For the feedback resistors, choose R1 = 80.6k. R2 can then
be calculated from equation (2) to be:
Figure 8 shows the complete circuit along with its effi-
ciency curve.
IN
C
22 F
16V
R
IN
V
2.85V TO
4.5V
will require an RMS current rating of at least 0.15A at
IN
2
††
V
0 8
OUT
.
1 1 171
100
90
80
70
60
50
OUT
R
1
V
L = 15 H
Burst Mode OPERATION
OUT
will require an ESR of less than
= 2.5V
OUTPUT CURRENT (mA)
k
V
IN
; use 169k
10
= 3.6V
V
IN
= 4.2V
100
1707 F08b
1000